Alpha acids are important molecules for a variety of purposes including as industrial chemicals, feed additives, therapeutics, and various other uses. Alpha acids include, for example, amino acids and alpha hydroxy acids. When two alpha acids react, they can form linear or cyclic dimers. When alpha acids react to form cyclic dimers the acid moiety is no longer free, and thus the cyclic dimers can have different physical properties such as reactivity, stability, and solubility, that can be advantageous for certain applications.
For example, cyclic dimers can be used as compounds capable of releasing alpha acids. Cyclic dimers of alpha acids also may provide important routes for synthesizing polymers and copolymers of alpha acids. For example, lactic acid cyclic dimers, also called lactides, provide an important route to polylactic acid, an important polymer which has attracted significant interest due to its properties of being biocompatible and biodegradable, and for its suitability for uses in the biomedical and industrial fields. Formation of lactide from lactic acid is complicated by competing oligomerization reactions. Most processes developed for producing lactide compounds involved treatment at high temperatures under a vacuum. For example, U.S. Pat. No. 5,274,073 describes production of lactide by evaporating water from lactic acid to give an oligomer, and then mixing the oligomer with a depolymerization catalyst followed by thermal cracking to produce the lactide as a vapor.
Unlike lactide, cyclic dimers of alpha acids with heteroatom side chain moieties have not been synthesized using the thermal cracking process. Heteroatom substituted cyclic dimers are desirable compounds because they can provide important functionalities for the numerous applications of the cyclic compounds. For example, substituted cyclic dimers may provide routes to functionalized and structurally diverse (i.e. branched, star, block) polymers and copolymers, which may have different or enhanced properties over polylactic acid. Other methods for synthesizing cyclic dimers have also failed for heteroatom containing alpha acids. A theoretical route to forming the cyclic compounds from reaction of the halogen substituted monomers give poor yields because the halogenated monomer is unstable. Thus, routes to functionalized cyclic dimers of alpha acids remain a synthetic challenge.
Thus, there is a need for heteroatom containing cyclic dimers as well as processes for making them.